Corrugated containerboard is manufactured on machines that combine one or more “liners” in a stack with fluted webs (“medium”) in between with the peaks of the medium flutes glued to the surfaces of the liners. The adhesive between the fluted medium and the liners of the combined board (that is, the corrugated containerboard) is then dried by passing the board through a double face heating section. The double face heating section (“double-backer”) consists of a series of steam-heated “steam chests” or “hot plates”. Individual steam chests and hot plates are generally less than two feet in machine direction length and extend to the width of the corrugator, which is typically 100 ″ to 120″ in width. The containerboard is held against these steam chests and hot plates by belts and ballast rollers that serve to keep the board in good thermal contact with the top surfaces of the hot plates/steam chests.
FIG. 1 shows a steam chest 10 according to the prior art. FIG. 2 shows a hot plate according to the prior art. Steam chests 10 and hot plates 40 are examples of steam heating devices designed to transfer heat from steam to a heating surface. Steam chests 10 can be constructed as large metal boxes 12 that are designed to hold the steam input at “A” the box interior 16 under pressure. The steam condenses on the top inside surface of the box 12 and the condensed steam (“condensate”) falls onto and collects on the bottom of the box 12. From there, the condensate is drained by gravity to a steam trap 22 from which the condensate is returned to the boiler at “B.” The box upper surface 14 is in contact with a containerboard 15 to be dried, which is held down to the upper surface 14 by a belt 20. Steam chests 10 are conventionally heated by steam that is supplied under pressure to each of the steam chests. The steam pressure to each group of steam chests 10 is typically controlled by a pressure control valve (not shown) working in conjunction with a pressure transmitter and a pressure indicating controller.
FIG. 2 shows the “hot plate” 40 (herein distinguished from the “steam chest” 10), which is similar in function to the steam chest 10, except the hot plate 40 has drilled internal passages 44 adjacent to a hot plate surface 46. The hot plate surface 46 and internal passages 44 are formed as part of a hot plate frame 42. These passages 44 generally extend from one side of the hot plate frame 42 to the opposite side, and then back again, forming several loops before the passage leaves the plate. The steam flows into inlet 43 and through these internal passages 44 and condenses as it transfers its heat to a corrugated containerboard 48 on the outside of surface 46. The condensate flows slowly by gravity toward a drain 45. The drain line is conventionally connected to a steam trap. Steam traps open to drain the condensate from the hot plate and then close to prevent the passage of uncondensed steam. The condensate that leaves the steam trap is returned to the boiler. At high condensing rates, the condensate that forms inside the passages of the hot plates 40 tends to accumulate and result in a reduction in rate and uniformity of heat transfer. The corrugated containerboard 48 can be held down to the hot plate surface 46 by a belt 50.
A typical corrugated containerboard making machine 300 with its associated double backer section 314 is shown in FIG. 3. The corrugated containerboard making machine 300 includes supply rollers 302 for the first liner, supply rollers 304 for the medium and supply rollers 306 for the second liner. The corrugated containerboard making machine 300 also includes a corrugator 308, drive rollers 310 and adhesive applicator 312. The corrugated containerboard making machine also includes a hot plates section 318 in a double backer section 314 for drying the adhesive applied at 312.
In order to minimize the non-uniformity of heat transfer, a multitude of hot plates are used in each double backer section 314. The pressure is adjusted on the belt 316 that holds the board to the hot plates 318 in an attempt to correct for these reductions in rate and uniformity of heat transfer. In conventional corrugators, the hot plate performance is controlled by the belt pressure, adding backing rolls, loading the backing rolls, increasing the steam pressure, venting some steam to atmosphere, adding more hot plates, or running the corrugating machine at a slower speed.
An example of prior art hot plates and their steam control system 400 are shown in FIG. 4. FIG. 4 shows a steam line 402 inputting steam at “A.” The steam line 402 delivers steam either directly to a hot plate 408 via delivery lines 414 or to pressure control valves 404, 406, which regulate the steam pressure and deliver steam to hot plates 410, 412 via delivery lines 416, 418. The steam heats the hot plates 408, 410, 412 and condenses, forming a condensate that is collected by condensate trap lines 420, 422, 424 and carried to separators 426, 428 which separate condensate from steam and return separated steam to the delivery lines 416, 418 or directly to a pump 432. Condensate is routed to pumps 430, 432 to be returned to the steam boiler (not shown) via a return line 434 at “B.”
The prior art hot plates and their steam systems are not suitable for high-speed corrugated boxboard production where uniformity, high heat transfer rates, and energy efficiency are important.